Problems due to overvoltages generated by lightnings particularly occur in such inverters to which DC current or DC voltage sources are connected which are exposed to the weather and which are thus prone to an increased danger of lightnings coupling-in. Besides photovoltaic generators this also applies to wind generators.
In an inverter described by HERNANDEZ J C ET AL: “Lightning and Surge Protection in Photovoltaic Installations” IEEE TRANSACTIONS ON POWER DELIVERY, IEEE SERVICE CENTER, NEW YORK, N.Y., US LNKD-DOI: 10.1109/TPWRD.2008.917886, Vol. 23, No. 4, Oct. 1, 2008 (2008 Oct. 1), pages 1961-1971, XP011225303ISSN: 0885-8977, each of two input lines of the inverter is connected to a grounded potential equalization busbar of the inverter via a varistor serving as a overvoltage eliminator. One of the two input lines may additionally be connected to the potential equalization busbar via a GFDI, i.e. a Ground Fault Detector and Interrupter, to apply ground potential to this input line, but to interrupt this ground connection automatically in case of a ground fault. If an overvoltage is coupled in the input lines of this known inverter by lightning, for example, it flows via the varistors to the potential equalization busbar and from there towards ground. So far as the varistors are not overloaded, they will again become high-ohmic, i.e. non-conductive after elimination of the overvoltage. If they are, however, overloaded, the varistors become permanently low-ohmic, i.e. conductive. If the varistors between both input lines and the potential equalization busbar are overloaded, this means an undesired short-circuit between the two input lines and thus of the DC power source connected to the input lines. Additionally, there is a considerable probability that this short-circuit does not result in the GFDI tripping, as it does not notice a ground fault due to the varistor connected in parallel which also became low-ohmic. Because of this probability, it is known to connect each of the two varistors in series with a fuse, which, however, means additional effort and related cost. Whereas varistors rarely include devices for fault signalling, this is regularly the case with GFDIs. Thus, tripping of a GFDI can easily be noticed and signalled as an indication of a fault of the inverter.
In another known inverter, two varistors are connected in series between its two input lines. An intermediate point between the two varistors is connected to a potential equalization busbar of this known inverter via an overvoltage eliminator which is based on a spark gap. The two varistors connected in series also allow for a compensation of overvoltages between the two input lines. Currents drained for overvoltage elimination towards the grounded potential equalization busbar flow across the spark gap which is turned off by the varistors becoming high-ohmic again after elimination of the overvoltage. If, however, one varistor becomes permanently low-ohmic due to an overload, turning the spark gap off is no longer ensured, and with overloading both varistors a permanent short circuit between the input lines arises.
For the purpose of lightning protection for inverters it is also known to provide lightning catching rods above a housing of the inverter, which are connected to a grounding device. The lightning catching rods are high voltage-isolated with regard to the housing of the inverter, and electric lines from the lightning catching rods to the grounding device also comprise a high voltage-isolation. Due to this high voltage-isolation, the characteristic wave resistance of this line for currents induced by a lightning is reduced so that these currents may drain to ground quickly without coupling into the inverter housing. However, a considerable danger exists that the ground currents flowing from the grounding device as a result of a lightning nevertheless couple into the inverter via the grounding of the potential equalization busbar of the inverter, as the ground currents may only flow off via a large ground area due to the high ground impedance.
There is a need for a power inverter and a solar plant including such a power inverter, which provide for an enhanced lightning protection at reduced cost.